Place peanut butter chips and 2/3 cup sweetened condensed milk in small microwave-safe bowl. Microwave at MEDIUM 1 to 1-1/2 minutes, stirring after 1 minute, until chips are melted and mixture is smooth when stirred. Stir in 1/2 teaspoon vanilla; spread evenly into prepared pan.
Place remaining sweetened condensed milk and chocolate chips in another small microwave-safe bowl; repeat above microwave procedure. Stir in remaining 1/2 teaspoon vanilla; spread evenly over peanut butter layer.
Cover; refrigerate until firm. Remove from pan; place on cutting board. Peel off foil. Cut into squares. Store tightly covered in refrigerator. About 4 dozen pieces or 1-1/2 pounds.
NOTE: For best results, do not double this recipe.